A diffuser having diffuser vanes is constituted by a side wall at a side plate end and another side wall at a core plate end of a radial impeller (hereinafter referred to simply as "impeller") and diffuser vanes. In a centrifugal type fluid machine having diffuser vanes, the diffuser is disposed such that the foregoing both side walls of the diffuser lie in a surface nearly perpendicular to a main shaft; and a radial diffuser is generally used, the radial diffuser allowing a high-velocity, radial, outward fluid which flows out of the impeller to flow out as it is in the radial direction. As the diffuser vanes, there are so-called "two-dimensional vanes" which have the same sectional shape from the side wall at the side plate end to the side wall at the core plate end constructing the diffuser. Ideally, a bent passage for guiding the fluid on the downstream side of the diffuser to a radial flow is formed to have a large radius of curvature. In a multi-stage centrifugal type fluid machine, however, allowing a large radius of curvature is disadvantageous in reducing the size of the machine in the direction of the radius and in the direction of the main shaft; therefore, the bent passage is composed of a rectangular passage which hardly has a radius of curvature.
The diffuser and the bent passage are good in the aspect of size reduction and economy of a fluid machine; on the other hand, however, a passage having an acute curve is disposed in the vicinity of an outlet of the diffuser and therefore, a force is applied to the curved portion in an oblique outward direction due to a change in the momentum which takes place when the upward flow in the radial direction is switched to a flow in the direction of the main shaft at the inlet of the bent passage. Hence, the flow in the vicinity of the outlet of the diffuser becomes a flow inclined toward the core plate rather than in the radial direction. As a result, the boundary layer of the surface along the side wall at the side plate end markedly develops, while the development of the boundary layer of the surface along the side wall at the core plate end becomes small because a main current approaches thereto. This trend becomes so marked that a handled flow rate is decreased below a design point, causing unstable characteristics (a drop in head-discharge curve) of a pump in a low flow rate zone. For this reason, a diffuser having so-called three-dimensional vanes has been developed to improve the unstable characteristics, to prevent a pump from becoming larger, and also to prevent pumping efficiency from being reduced, the diffuser having been disclosed in Japanese Patent Unexamined Publication No. 61-258998.
The diffuser vanes are shaped so that the spreading angle between the vanes is made smaller in a portion near the side wall at the side plate end, while the spreading angle between the vanes in a portion near the side wall at the core plate end is made larger. Hence, a rise in pressure due to the deceleration of the main stream flowing in the vicinity of the side wall of the side plate of the diffuser decreases so as to control the boundary layer which develops along the side wall at the core plate end, and the loss attributable to a secondary flow of the boundary layer is decreased and the separation of the flow on the side wall at the side plate end which takes place in the low flow rate zone is prevented.
In the diffusers having the two-dimensional or three-dimensional vanes, no consideration has been given to the flow of a fluid in the diffuser in the low flow rate zone or to the flow of a fluid which comes out of a multi-stage diffuser and passes through a bent passage. Regarding a flow on the downstream side of the diffuser, the flow passing the side plate of the diffuser moves along the outer side wall surface of the bent passage, therefore, the distance before reaching a return passage located further at the downstream side of the bent passage is prolonged, leading to an increased friction loss. The flow of the fluid passing the core plate of the diffuser moves along the inner side wall surface of the bent passage; the flow cannot move along the wall surface in the bent portion having a small radius of curvature and it separates, making it easy for the boundary layer to develop on the inner wall surface.
Thus, the loss due to the friction or separation deteriorates the performance of a centrifugal type fluid machine, and since the flow velocity of the fluid in a diffuser decreases in the low flow rate zone, the spreading angle between the vanes becomes relatively large; therefore, the fluid has been apt to flow back and the head-discharge curve has been exhibiting unstable characteristics.
An object of the present invention is to provide a centrifugal type fluid machine which permits higher efficiency by reducing the flow loss in a bent passage on the downstream side of a diffuser and by reducing the flow loss in the diffuser in a low flow rate zone in a multi-stage type one, and also by reducing the flow loss in the diffuser in the low flow rate zone in a single-stage type one.